1. Field of the Invention
This invention relates to ring resonators, and more particularly to an apertureless ring laser gyroscope capable of suppressing all transverse modes except the fundamental Gaussian mode (TEM.sub.00).
2. Description of the Related Art
Modern inertial navigation includes rotational sensors capable of wide dynamic rate sensing range, high reliability, and rapid reaction time. Mechanical gyroscopes meet some of these capabilities, but exhibit problems associated with shock and vibration. Additionally, mechanical gyroscopes have relatively long warm-up times and limited high rate sensing capability. Ring laser gyroscopes have been developed which constitute the laser analog of the Sagnac experiments for detecting rotation rate by means of counter propogating light beams.
Ring laser gyroscopes have an optical pathway arranged in a polygonal configuration defined by three or more mirrors. The optical pathway may be confined to a cavity defined within a monolithic glass block frame. FIG. 1 shows an example of a monolithic block ring laser gyroscope frame 10 which defines an optical pathway through cavity legs 12, 14, 16, and 18. Measurements of rotation rate are made (in this prior art gyroscope) by using the lasing characteristics of an optical cavity. The cavity legs 12, 14, 16 and 18 are filled with a gaseous active laser medium such as a mixture of Helium and Neon. Usually, the gaseous laser medium is maintained at a pressure of no more than 7-11 torr. Four mirrors, 20, 22, 24 and 26, are sealed to four mirror mounting surfaces of the frame 10 in a typical embodiment shown in FIG. 1. The gyroscope frame 10 may be mounted onto a mounting post 28 made of a strong and resilient metal such as INVAR.
The laser gyroscope of FIG. 1 may include a group of elements for exciting the active medium. These may include a cathode 30 and a set of anodes 32 and 34, symmetrically positioned about the cathode 30, forming the DC discharge system. The active region between anode 34 and cathode 30, as well as the active region between anode 32 and the cathode 30, comprise the excited active medium source that is used to stimulate the lasing action needed to create at least two counter propagating longitudinal modes of light needed to measure rotation. An output prism 36 may be used to optically heterodyne counter propagating modes in order to measure rotation.
It will be noted that in a prior art gyroscope, as shown in FIG. 1, at least one leg 12 of the optical pathway cavity has a restricted aperture 38. The purpose of this aperture 38 is to suppress all higher order transverse modes other than the dominant Gaussian TEM.sub.00 mode. (When referred to in this application the TEM.sub.00 mode is also referred to as the fundamental or Gaussian mode).
It is heretofore known that, to achieve optimum performance, a laser cavity of a ring laser gyroscope should be designed, constructed, and aligned which supports only one transverse mode. This is true because the beam should have only one intensity maximum in its energy cross-section. Oscillation is desired in a single transverse mode, such as the gaussian or the lowest order mode (TEM.sub.00) . In the prior art (FIG. 1) an aperture 38 is used suppress all modes but the one transverse TEM.sub.00 mode that is present in the optical cavity 12. This mode is characterized by minimal cross-section at the mirrors and other optical elements creating minimal backscatter. In order that the laser oscillates in the fundamental TEM.sub.00 mode, a limiting aperture 38 is introduced in the ring resonator so that all modes, except TEM.sub.00, have diffraction losses which overcome the available gain of the active medium. The operation of the aperture 38 of FIG. 1 is better understood by reference to FIG. 2, which is a prior art graphic representation illustrative of gain plotted as a function of frequency. The fundamental TEM.sub.00 transverse mode is shown to have a gain curve 50 which exists substantially above the gain threshold line 54, allowing this TEM.sub.00 transverse mode to lase at its resonant frequency 58. This TEM.sub.00 curve 50 is characterized by hole-burning at the lasing frequency 58.
The aperture 38 (FIG. 1) causes a differential loss between the gain threshold lines 56 and 54. Line 56 is associated with the higher order mode TEM.sub.01, and the gain curve 52 representing that higher order TEM.sub.01 mode. It will be noted that gain curve 52 is entirely below its gain threshold line 56 and therefore the ring laser gyroscope does not support lasing at this off-axis mode. Thus, the conventional aperture 38 does achieve tranverse mode suppression, in the manner described herein and illustrated in FIG. 2.
Although a conventional aperture 38 in the optical cavity 12 (FIG. 1) achieves tranverse mode suppression, it does so with certain costs to gyroscope performance. Foremost among the aperture effects which reduce gyroscope performance is the increased loss and scatter in the optical cavity. It is well-known that an aperture positioned within the optical cavity of a ring laser gyroscope contributes to both forward scatter and backscatter between the two counter-propagating modes within the ring resonator cavity. Backscatter is detrimental to gyroscope performance, since it is a source of gyro scale factor nonlinearities. Thus, a transverse mode suppression technique and structure which eliminates the aperture, would be useful to reduce scatter.
Additionally, an aperture 38 as shown along the cavity 12 of the gyroscope frame 10 (FIG. 1) is difficult to fabricate, in light of the severe tolerances required during the ring laser gyroscope manufacturing process. Difficult fabrication also is indicative of higher cost. Thus, it would be useful if another mechanism could be found which could achieve higher order transverse mode suppression without the defects and drawbacks of the conventional aperture 38.
One attempt at suppressing undesired modes is illustrated in U.S. Pat. Nos. 4,519,708 and 4,627,732, both assigned to Raytheon Company of Lexington, Mass. In these patents, an aperture is formed on the surface of the mirror using a dielectric material comprising a plurality of layers which purport to absorb light waves and scatter. However, these patents disclose a system which necessarily makes at least some contribution to scatter in the optical cavity and therefore represent a partial solution to the problem of unwanted scatter. Another patent taught placing a device in the optical pathway of a ring laser gyrosocope for purposes of light absorption (U.S. Pat. No. 4,494,873, also assigned to Raytheon Company). Additionally, investigations have been made as to the effect of gain saturation on the oscillating modes of optical resonant cavities. See "Effect of Gain Saturation on the Oscillating Modes of Optical Mazers" by Fox and Li, IEEE Journal of Quantum Electronics, Vol. QE-2, No. 12, page 774-783 (December 1966).